It is well known that hydrocarbons (e.g. crude oil and natural gas) are recovered from subterranean formations by drilling a wellbore into the subterranean reservoirs where the hydrocarbons reside, and using the natural pressure of the hydrocarbon or other lift mechanism such as pumping, gas lift, electric submersible pumps (ESP) or another mechanism or principle to produce the hydrocarbons from the reservoir. Conventionally most hydrocarbon production is accomplished using a single wellbore. However, techniques have been developed using multiple wellbores, such as the secondary recovery technique of water flooding, where water is injected into the reservoir to displace oil. The water from injection wells physically sweeps the displaced oil to adjacent production wells. Potential problems associated with water flooding techniques include inefficient recovery due to variable permeability or similar conditions affecting fluid transport within the reservoir. Early breakthrough is a phenomenon that may cause production and surface processing problems.
Hydraulic fracturing is the fracturing of subterranean rock by a pressurized liquid, which is typically water mixed with a proppant (often sand) and chemicals. The fracturing fluid is injected at high pressure into a wellbore to create, in shale for example, a network of fractures in the deep rock formations to allow hydrocarbons to migrate to the well. When the hydraulic pressure is removed from the well, the proppants, e.g. sand, aluminum oxide, etc., hold open the fractures once fracture closure occurs. In one non-limiting embodiment chemicals are added to increase the fluid flow and reduce friction to give “slickwater” which may be used as a lower-friction-pressure placement fluid. Alternatively in different non-restricting versions, the viscosity of the fracturing fluid is increased by the addition of polymers, such as crosslinked or uncrosslinked polysaccharides (e.g. guar gum) or by the addition of viscoelastic surfactants (VES).
Recently the combination of directional drilling and hydraulic fracturing has made it economically possible to produce oil and gas from new and previously unexploited ultra-low permeability hydrocarbon bearing lithologies (such as shale) by placing the wellbore laterally so that more of the wellbore, and the series of hydraulic fracturing networks extending therefrom, is present in the production zone permitting more production of hydrocarbons as compared with a vertically oriented well that occupies a relatively small amount of the production zone. “Laterally” is defined herein as a deviated wellbore away from a more conventional vertical wellbore by directional drilling so that the wellbore can follow the oil-bearing strata that are oriented in a non-vertical plane or configuration. In one non-limiting embodiment, a lateral wellbore is any non-vertical wellbore. In another non-limiting embodiment, a lateral wellbore is defined as any wellbore that is at an inclination angle from vertical ranging from about 45° to about 135°. It will be understood that all wellbores begin with a vertically directed hole into the earth, which is then deviated from vertical by directional drilling such as by using whipstocks, downhole motors and the like. A wellbore that begins vertically and then is diverted into a generally horizontal direction may be said to have a “heel” at the curve or turn where the wellbore changes direction and a “toe” where the wellbore terminates at the end of the lateral or deviated wellbore portion. The “sweet-spot” of the hydrocarbon bearing reservoir is an informal term for a desirable target location or area within an unconventional reservoir or play that represents the best production or potential production. The combination of directional drilling and hydraulic fracturing has led to the so-called “fracking boom” of rapidly expanding oil and gas extraction in the US beginning in about 2003.
Improvements are always needed in the driller's ability to find and map sweet-spots to enable wellbores to be placed in the most productive areas of the reservoirs. Sweet-spots in shale reservoirs may be defined by the source rock richness or thickness, by natural fractures present therein or by other factors. Conventionally, geological data, e.g. core analysis, well log data, seismic data and combinations of these are used to identify sweet-spots in unconventional plays.